Study Of Flow And Heat Transfer Mechanisms During Close-contact Melting Of Phase Change Materials

Latent heat thermal energy storage(LHTES)technology has broad application prospects in areas such as industrial waste heat recovery,renewable energy utilization,low-carbon buildings,and thermal management of electronic devices.However,the low thermal conductivity of lowtemperature phase change materials(PCM)like paraffin severely affects the heat response rate and power density of thermal storage systems,limiting the application and development of LHTES.The close-contact melting(CCM)process is a special melting form dominated by microliquid film heat transfer.The extremely thin liquid film thickness in molten phase change materials reduces the overall thermal resistance,allowing the construction of a CCM region to significantly reduce heat transfer resistance.This offers a new way to enhance the solid-liquid phase change heat transfer process,beyond the traditional approach of improving the thermal conductivity of phase change materials.Therefore,fully utilizing the CCM mechanism in different application scenarios is a key step to overcome the low power density bottleneck of mid-low temperature LHTES.It represents a highly promising and feasible approach to achieve rapid charging of thermal storage systems.By leveraging the advantages of CCM and exploring its potential in various practical contexts,significant advancements can be made in the efficiency and performance of LHTES technology.In-depth study and understanding of the flow and heat transfer mechanism in CCM are necessary prerequisites and important foundations for effectively utilizing this mechanism.However,previous theoretical and experimental studies have mainly focused on the macroscopic melting behavior of CCM,lacking measurements of the thickness variation of the micro-liquid film and a comprehensive understanding of its internal flow and heat transfer characteristics.Additionally,existing theoretical models fail to elucidate the effects of factors such as high degrees of superheat,non-Newtonian rheological properties,composite phase change materials,and different geometric configurations on solid-liquid phase change heat transfer in CCM.The present study first designs and constructs an experimental setup for measuring the thickness of micro-liquid films during the CCM process,based on the laser interference principle.This setup enables precise in-situ measurement of the micro-liquid film thickness variation.Theoretical analysis is performed to establish a predictive model for transient changes in the liquid film thickness,and the experimental results validate the model.The combined experimental and theoretical findings reveal that the liquid film thickness in the CCM process undergoes a temporal evolution characterized by “initial rapid growth,long-term quasi-steady-state slow growth,and final rapid growth”.This behavior is observed under various conditions of superheat and initial size using tetradecanol as a typical phase change material,with the quasisteady-state thickness of the liquid film found to be in the range of hundreds of micrometers(approximately 200 μm).Subsequently,this study establishes a shear-thinning fluids model for CCM considering the convective effect within the liquid film based on two rheological models: Carreau and powerlaw.The experimental measurements using the laser interference setup on self-made shearthinning samples validate the reliability of model.The theoretical model reveals a key parameter,the critical liquid film thickness,in the Carreau PCM during the CCM process.Based on the relationship between the growth of the liquid film thickness and the relative size of the critical liquid film thickness(always smaller,always larger,or initially smaller and then larger),three different control modes are identified.Furthermore,under the approximation and simplification assumptions,explicit analytical solutions for the Carreau and power-law PCM are derived.The non-dimensional parameter space is determined for the validity of approximate analytical solution of the Carreau model,and the parameter conditions are identified for its approximation to the power-law model.Additionally,both experimental and theoretical findings demonstrate that the convective effect within the liquid film cannot be neglected under high degrees of superheat conditions(with Stefan number Ste > 0.1).Assuming pure conduction would overestimate the melting and heat transfer rates predicted by the model.The comparison between theoretical and experimental results confirms that using an exponential temperature distribution assumption within the liquid film effectively reduces prediction deviations.A modification method is proposed,which involves replacing the Ste term with ln(Ste+1)in the non-dimensional equations or analytical solutions to improve the predictions.In order to investigate the feasibility of synergistic heat transfer enhancement through the addition of nanoparticles in CCM,this study experimentally and theoretically investigates composite PCM with different concentrations of graphene nanoplatelets loaded in tetradecanol.It is demonstrated that the nanocomposite PCM can be treated as a homogeneous material during the flow and heat transfer process of CCM.Both experimental and theoretical results of this study indicate that the addition of 1 wt.% nanoparticles can increase the melting rate,while the addition of 3 wt.% nanoparticles actually decreases the melting rate.This is attributed to the fact that addition of high concentrations of nanoparticles leads to a significant increase in material viscosity and a notable decrease in phase change enthalpy.These effects result in an increase in the liquid film thickness within the CCM region and a decrease in the thermal energy storage capacity of the material,outweighing the positive effect of increased thermal conductivity.Computational analysis of the heat storage rate reveals that due to the comprehensive changes in thermal properties induced by the addition of nanoparticles,a high melting rate does not necessarily imply a high heat storage rate.Therefore,in order to achieve synergistic heat transfer enhancement through the combination of CCM and nanocomposite PCM,it is necessary to consider the simultaneous improvement of the material properties of the nanocomposite PCM and the design of operating conditions.Subsequently,this study employed the volume expansion method to measure the overall heat transfer performance of nanocomposite PCM and developed a theoretical model for CCM in a spherical container.Due to the simultaneous presence of the bottom CCM heat transfer region and the top natural convection heat transfer region within the spherical container,the addition of nanoparticles affects the heat transfer characteristics of both regions.Although the increased thermal conductivity of the composite PCM enhances the heat transfer performance of both the CCM region and the natural convection region,the sharp increase in material viscosity results in an increase in liquid film thickness in the CCM region and an increase in flow resistance in the natural convection region.Theoretical analysis reveals that the influence of superheat and container size on the heat transfer of two regions differs: increasing the superheat and container size leads to an increase in liquid film thickness in the CCM region,thereby increasing the equivalent thermal resistance and deteriorating heat transfer; whereas increasing the superheat and container size enhances the flow intensity in the natural convection region,achieving improved heat transfer performance.Within the range of conditions studied in this paper,the 3 wt.% samples exhibited heat transfer deterioration,while the 1 wt.% samples achieved heat transfer enhancement only under specific combinations of superheat degree and container size.Therefore,for specific operating superheat degree and sphere diameter sizes,it is necessary to match an appropriate concentration of added nanoparticles,taking into account their thermal property parameters and temperature dependency,to further enhance heat transfer.Lastly,to investigate the dimension and size effects of CCM and differentially-heated melting processes in rectangular enclosures exhibiting translational similarity,this study derived and established approximate analytical solutions for CCM and scaling laws for differentially-heated melting in finite rectangular sections.The results indicate that the CCM process with a rectangular section can be approximated as a two-dimensional process only when the aspect ratio of the bottom section is less than 0.01 or greater than 100.Furthermore,when the bottom area remains constant,the configuration with an aspect ratio of 1 exhibits the smallest liquid film thickness and the fastest melting rate during CCM.Regarding the differentially-heated melting process,the results show inherent differences between two-dimensional and three-dimensional simulations that cannot be eliminated by adjusting simulation parameters.The scaling law analysis reveals that dimensional reduction leads to a decrease in the dimensional constants and the disappearance of size ratio parameters in the scaling relationships for the transitional and convective stages,resulting in an overestimation of melting and heat transfer rates.The scaling law analysis and numerical simulation results collectively demonstrate that the differentially-heated melting process can be simplified into a two-dimensional problem only when the Prandtl number of the PCM is much greater than 1 and the aspect ratio of the rectangular enclosure is much smaller than 1.